IDR Working Group                                         R. Raszuk, Ed.
Internet-Draft                                   NTT Network Innovations
Intended status: Standards Track                               C. Cassar                        B. Decraene, Ed.
Expires: December 2, 16, 2021                                        Orange
                                                               C. Cassar

                                                                 E. Aman

                                                        B. Decraene, Ed.
                                                                  Orange

                                                                 K. Wang
                                                        Juniper Networks
                                                            May 31,
                                                           June 14, 2021

                 BGP Optimal Route Reflection (BGP-ORR)
             draft-ietf-idr-bgp-optimal-route-reflection-24
             draft-ietf-idr-bgp-optimal-route-reflection-25

Abstract

   This document defines an extension to BGP route reflectors.  On route
   reflectors, BGP route selection is modified in order to choose the
   best route from the standpoint of their clients, rather than from the
   standpoint of the route reflectors.  Depending on the scaling and
   precision requirements, route selection can be specific for one
   client, common for a set of clients or common for all clients of a
   route reflector.  This solution is particularly applicable in
   deployments using centralized route reflectors, where choosing the
   best route based on the route reflector's IGP location is suboptimal.
   This facilitates, for example, best exit point policy (hot potato
   routing).

   The solution relies upon all route reflectors learning all paths
   which are eligible for consideration.  BGP Route Selection is
   performed in the route reflectors based on the IGP cost from
   configured locations in the link state IGP.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on December 2, 16, 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Modifications to BGP Route Selection  . . . . . . . . . . . .   4
     3.1.  Route Selection from a different IGP location . . . . . .   5
       3.1.1.  Restriction when BGP next hop is a BGP prefix . . . .   6
     3.2.  Multiple Route Selections . . . . . . . . . . . . . . . .   6
   4.  Deployment Considerations . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   There are three types of BGP deployments within Autonomous Systems
   today: full mesh, confederations and route reflection.  BGP route
   reflection [RFC4456] is the most popular way to distribute BGP routes
   between BGP speakers belonging to the same Autonomous System.
   However, in some situations, this method suffers from non-optimal
   path selection.

   [RFC4456] asserts that, because the IGP cost to a given point in the
   network will vary across routers, "the route reflection approach may
   not yield the same route selection result as that of the full IBGP
   mesh approach."  One practical implication of this assertion is that
   the deployment of route reflection may thwart the ability to achieve
   hot potato routing.  Hot potato routing attempts to direct traffic to
   the closest AS exit point in cases where no higher priority policy
   dictates otherwise.  As a consequence of the route reflection method,
   the choice of exit point for a route reflector and its clients will
   be the exit point that is optimal for the route reflector - not
   necessarily the one that is optimal for its clients.

   Section 11 of [RFC4456] describes a deployment approach and a set of
   constraints which, if satisfied, would result in the deployment of
   route reflection yielding the same results as the IBGP full mesh
   approach.  This deployment approach makes route reflection compatible
   with the application of hot potato routing policy.  In accordance
   with these design rules, route reflectors have often been deployed in
   the forwarding path and carefully placed on the POP to core
   boundaries.

   The evolving model of intra-domain network design has enabled
   deployments of route reflectors outside of the forwarding path.
   Initially this model was only employed for new services, e.g. e.g., IP
   VPNs [RFC4364], however it has been gradually extended to other BGP
   services
   services, including the IPv4 and IPv6 Internet.  In such
   environments, hot potato routing policy remains desirable.

   Route reflectors outside of the forwarding path can be placed on the POP
   to core boundaries, but they are often placed in arbitrary locations
   in the core of large networks.

   Such deployments suffer from a critical drawback in the context of
   BGP Route Selection: A route reflector with knowledge of multiple
   paths for a given prefix will typically pick its best path and only
   advertise that best path to its clients.  If the best path for a
   prefix is selected on the basis of an IGP tie-break, the path
   advertised will be the exit point closest to the route reflector.
   However, the clients are in a different place in the network topology
   than the route reflector.  In networks where the route reflectors are
   not in the forwarding path, this difference will be even more acute.

   In addition, there are deployment scenarios where service providers
   want to have more control in choosing the exit points for clients
   based on other factors, such as traffic type, traffic load, etc.
   This further complicates the issue and makes it less likely for the
   route reflector to select the best path from the client's
   perspective.  It follows that the best path chosen by the route
   reflector is not necessarily the same as the path which would have
   been chosen by the client if the client had considered the same set
   of candidate paths as the route reflector.

2.  Terminology

   This memo makes use of the terms defined in [RFC4271] and [RFC4456].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Modifications to BGP Route Selection

   The core of this solution is the ability for an operator to specify
   the IGP location for which the route reflector calculates interior
   cost for the NEXT_HOP.  The IGP location is defined as a node in the
   IGP topology, it is identified by an IP address of this node (e.g. (e.g., a
   loopback address), and may be configured on a per route reflector
   basis, per set of clients, or per client basis.  This ability enables
   the route reflector to send to a given set of clients routes with
   shortest distance to the next hops from the position of the selected
   IGP location.  This provides for freedom of route reflector physical
   location, and allows transient or permanent migration of this network
   control plane function to an arbitrary location.

   The choice of specific granularity (route reflector, set of clients,
   or client) is configured by the network operator.  An implementation
   is considered compliant with this document if it supports at least
   one listed grouping of IGP location.

   For purposes of route selection, the perspective of a client can
   differ from that of a route reflector or another client in two
   distinct ways:

   o  it has a different position in the IGP topology, and

   o  it can have a different routing policy.

   These factors correspond to the issues described earlier.

   This document defines, for BGP Route Reflectors [RFC4456], two
   changes to the BGP Route Selection algorithm:

   o  The first change, introduced in Section 3.1, is related to the IGP
      cost to the BGP Next Hop in the BGP decision process.  The change
      consists in of using the IGP cost from a different IGP location than
      the route reflector itself.

   o  The second change, introduced in Section 3.2, is to extend the
      granularity of the BGP decision process, to allow for running
      multiple decisions processes using different perspective or
      policies.

   A significant advantage of these approaches is that the route
   reflector clients do not need to be modified.

3.1.  Route Selection from a different IGP location

   In this approach, optimal refers to the decision where the interior
   cost of a route is determined during step e) of [RFC4271] section
   9.1.2.2 "Breaking Ties (Phase 2)".  It does not apply to path
   selection preference based on other policy steps and provisions.

   In addition to the change specified in [RFC4456] section 9, [RFC4271]
   section 9.1.2.2 is modified as follows.

   The below text in step e)

      e) Remove from consideration any routes with less-preferred
      interior cost.  The interior cost of a route is determined by
      calculating the metric to the NEXT_HOP for the route using the
      Routing Table.

   ...is replaced by this new text:

      e) Remove from consideration any routes with less-preferred
      interior cost.  The interior cost of a route is determined by
      calculating the metric from the selected IGP location to the
      NEXT_HOP for the route using the shortest IGP path tree rooted at
      the selected IGP location.

   In order to be able to compute the shortest path tree rooted at the
   selected IGP locations, knowledge of the IGP topology for the area/
   level that includes each of those locations is needed.  This
   knowledge can be gained with the use of the link state IGP such as
   IS-IS [ISO10589] or OSPF [RFC2328] [RFC5340] or via BGP-LS [RFC7752].
   One or more backup IGP locations SHOULD be allowed to be specified
   for redundancy.

3.1.1.  Restriction when BGP next hop is a BGP prefix

   In situations where the BGP next hop is a BGP prefix itself, the IGP
   metric of a route used for its resolution SHOULD be the final IGP
   cost to reach such next hop.  Implementations which can not cannot inform BGP
   of the final IGP metric to a recursive next hop MUST treat such paths
   as least preferred during next hop metric comparison.  However  However, such
   paths MUST still be considered valid for BGP Phase 2 Route Selection.

3.2.  Multiple Route Selections

   BGP Route Reflector as per [RFC4456] runs a single BGP Decision
   Process.  Optimal route reflection may require multiple BGP Decision
   Processes or subsets of the Decision Process in order to consider
   different IGP locations or BGP policies for different sets of
   clients.

   If the required routing optimization is limited to the IGP cost to
   the BGP Next-Hop, only step e) and below as defined [RFC4271] section
   9.1.2.2, needs to be run multiple times.

   If the routing optimization requires the use of different BGP
   policies for different sets of clients, a larger part of the decision
   process needs to be run multiple times, up to the whole decision
   process as defined in section 9.1 of [RFC4271].  This is for example
   the case when there is a need to use different policies to compute
   different degree of preference during Phase 1.  This is needed for
   use cases involving traffic engineering or dedicating certain exit
   points for certain clients.  In the latter case, the user may specify
   and apply a general policy on the route reflector for a set of
   clients.  Regular path selection, including IGP perspective for a set
   of clients as per Section 3.1, is then applied to the candidate paths
   to select the final paths to advertise to the clients.

   A route reflector can implement either or both of the modifications
   in order to allow it to choose the best path for its clients that the
   clients themselves would have chosen given the same set of candidate
   paths.

4.  Deployment Considerations

   BGP Optimal Route Reflection provides a model for integrating the
   client perspective into the BGP Route Selection decision function for
   route reflectors.  More specifically, the choice of BGP path factors
   in either the IGP cost between the client and the NEXT_HOP (rather
   than the IGP cost from the route reflector to the NEXT_HOP) or other
   user configured policies.

   The achievement of optimal routing between clients of different
   clusters relies upon all route reflectors learning all paths that are
   eligible for consideration.  In order to satisfy this requirement,
   BGP add-path [RFC7911] needs to be deployed between route reflectors.

   This solution can be deployed in traditional hop-by-hop forwarding
   networks as well as in end-to-end tunneled environments.  In networks
   where there are multiple route reflectors and hop-by-hop forwarding
   without encapsulation, such optimizations SHOULD be consistently
   enabled in a
   consistent way on all route reflectors.  Otherwise, clients may receive an
   inconsistent view of the network, in turn leading to intra-domain
   forwarding loops.

   As discussed in section 11 of [RFC4456], the IGP locations of BGP
   route reflectors is important and has routing implications.  This
   equally applies to the choice of the IGP locations configured on
   optimal route reflectors.  If a backup location is provided, it is
   used when the primary IGP location disappears from the IGP (i.e.
   fails).  Just like the failure of a RR [RFC4456], it may result in
   changing the paths selected and advertised to the clients and in
   general the post-failure paths are expected to be less optimal.  This
   is dependent on the IGP topologies and the IGP distance between the
   primary and the backup IGP locations: the smaller the distance the
   smaller the potential impact.

   After selecting suitable IGP locations, an operator may let one or
   multiple route reflectors handle route selection for all of them.
   The operator may alternatively deploy one or multiple route reflector
   for each IGP location or create any design in between.  This choice
   may depend on operational model (centralized vs per region),
   acceptable blast radius in case of failure, acceptable number of IBGP
   sessions for the mesh between the route reflectors, performance and
   configuration granularity of the equipment.

   With this approach, an ISP can effect a hot potato routing policy
   even if route reflection has been moved out of the forwarding plane,
   and hop-by-hop switching has been replaced by end-to-end MPLS or IP
   encapsulation.  Compared with a deployment of ADD-PATH on all
   routers, BGP ORR reduces the amount of state which needs to be pushed
   to the edge of the network in order to perform hot potato routing.

   Modifying the IGP location of BGP ORR does not interfere with
   policies enforced before IGP tie-breaking (step e) in the BGP
   Decision Process Route.

   Calculating routes for different IGP locations requires multiple SPF
   calculations and multiple (subsets of) BGP Decision Processes, which
   requires more computing resources.  This document allows for
   different granularity such as one Decision Process per route
   reflector, per set of clients or per client.  A more fine grained fine-grained
   granularity may translate into more optimal hot potato routing at the
   cost of more computing power.  Selecting to configure an IGP location
   per client has the highest precision as each client can be associated
   with their ideal (own) IGP location.  However, doing so may have an
   impact on the performance (as explained above).  Using an IGP
   location per set of clients implies a loss of precision, but reduces
   the impact on the performance of the route reflector.  Similarly, if
   an IGP location is selected for the whole routing instance, the
   lowest precision is achieved achieved, but the performance impact is minimal
   (both should be equal to the [RFC4456] ones).  The ability to run
   fine grained
   fine-grained computations depends on the platform/hardware deployed,
   the number of clients, the number of BGP routes and the size of the
   IGP topology.  In essence, sizing considerations are similar to the
   deployments of BGP Route Reflector.

5.  Security Considerations

   Similarly to [RFC4456], this

   This extension to provides a new metric value using additional
   information for computing routes for BGP router reflectors.  While
   any improperly used metric value could impact the resiliency of the
   network, this extension does not change the underlying security
   issues inherent in the existing IBGP. IBGP per [RFC4456].

   This document does not introduce requirements for any new protection
   measures.

6.  IANA Considerations

   This document does not request any IANA allocations.

7.  Acknowledgments

   Authors would like to thank Keyur Patel, Eric Rosen, Clarence
   Filsfils, Uli Bornhauser, Russ White, Jakob Heitz, Mike Shand, Jon
   Mitchell, John Scudder, Jeff Haas, Martin Djernaes, Daniele
   Ceccarelli, Kieran Milne, Job Snijders, Randy Bush and Bush, Alvaro Retana and
   Lars Eggert for their valuable input.

8.  Contributors

   Following persons substantially contributed to the current format of
   the document:

   Stephane Litkowski
   Cisco System

   slitkows.ietf@gmail.com

   Adam Chappell
   GTT Communications, Inc.
   Aspira Business Centre
   Bucharova 2928/14a
   158 00 Prague 13 Stodulky
   Czech Republic

   adam.chappell@gtt.net

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
              <https://www.rfc-editor.org/info/rfc4456>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [ISO10589]
              International Organization for Standardization,
              "Intermediate system to Intermediate system intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)", ISO/
              IEC 10589:2002, Second Edition, Nov 2002.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,
              <https://www.rfc-editor.org/info/rfc7911>.

Authors' Addresses

   Robert Raszuk (editor)
   NTT Network Innovations

   Email: robert@raszuk.net

   Bruno Decraene (editor)
   Orange

   Email: bruno.decraene@orange.com

   Christian Cassar

   Email: cassar.christian@gmail.com

   Erik Aman

   Email: erik.aman@aman.se

   Bruno Decraene (editor)
   Orange

   Email: bruno.decraene@orange.com
   Kevin Wang
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   USA

   Email: kfwang@juniper.net